Method of preparing polyamide acid for processing of semiconductors

ABSTRACT

An improved method of preparing polyamide acid for processing of semiconductors from a diamine and a tetracarboxylic acid dianhydride with a diaminocarbonamide, is provided (1) by reacting the components at below 40° C. until the reduced viscosity reaches above 0.5 dl/g. at 30° C., and then, (2) by adjusting the reduced viscosity to more than 0.3 dl/g, at 30°0 C. and the solution viscosity to 500-3,000 cps at 25° C. by heating at 50°-100° C. By applying the polyamide acid obtained by the method to semiconductor apparatus, they get sufficient heat-resistance without pin holes, and their reliability may be greatly improved.

This is a division of application Ser. No. 307,199 filed Sept. 30, 1981,now U.S. Pat. No. 4,447,596 which in turn is a continuation of Ser. No.54,429, filed July 3, 1979, now abandoned.

BACKGROUND OF THE INVENTION

The invention relates to a method of preparing a polyamide acid, usedfor processing of semiconductors, obtained from a diamine and atetracarboxylic acid dianhydride, optionally with a diaminocarbonamidecompound.

More particularly, it is to provide a polyamide acid for processing ofsemiconductors capable of transforming into a polyimide resin, havingimide rings and/or benzoylenequinazolone rings, useful as insulatingfilms for semiconductors such as a surface-protecting film ofsemiconductors or layer-insulating film of semiconductors havingmulti-layer construction.

Heretofore, silicon dioxide films which were formed by the chemicalgas-phase growing method have mainly been employed as insulating filmsfor semiconductors, such as surface-protecting films of semiconductorsor layer-insulating films of wired conductors having multi-layer wiringconstruction.

However, silicon dioxide films have defects. There is a tendency tocrack due to stress of films if they are thick in size or if they areused for layer-insulating films of a wired conductor having more thanthree layers. Snapping of wired conductors also tends to occur due totheir poor step coverage.

These defects have prevented improvements in efficiency and integrationof semiconductors.

In order to overcome these defects, there have been recently employed,as insulating films, organic materials, particularly polyimide groupresins having imide rings and/or benzoylenequinazolone rings obtained byheating and dehydrating a polyamide acid synthesized from a diamine anda tetracarboxylic acid dianhydride optionally with a diaminocarbonamide.Such films have superior heat-resistance.

By using the materials, the defects of silicon dioxide films such asoccurrence of cracks and poor step coverage have been avoided, and thusnovel semiconductor apparatus have been invented as disclosed inJapanese patent publications Nos. 51-44871/1976 and 51-31185/1976 andthe respective corresponding U.S. Pat. No. 3,846,166 and No. 4,017,886.

The above-mentioned insulating film of a polyimide group resin havingimide rings and/or benzoylenequinazolone rings may be formed bypreparing a polyamide acid from a diamine and a tetracarboxylic aciddianhydride optionally with a diaminocarbonamide, then making a solutionof the thus obtained polyamide acid with an inert solvent, thereaftercoating the solution on a wafer with a photoresist rotatory coatingmachine, and finally heating the coated wafer.

The insulating film must be formed uniformly with a thickness of below 5μM and within the range of ±0.2 μM scattering, and without any pin holescaused by runaway (repelling) of coated film from viewpoint of thecapabilities of semiconductors.

It must also have heat-resistance enough for withstandingheat-processing applied during production of semiconductors, such aswire bonding and contact alloy processing between electrodes.

However, there have taken place such problems in the course of trialmanufacture of many semiconductors, in which the above-mentionedpolyamide acid was used, that a uniform film with sufficient thicknesscould not be formed. Pin holes due to runaway of coated film developedand the desired heat-resistance could not be obtained.

BRIEF SUMMARY OF THE INVENTION

It is, therefore, an object of the invention to provide a polyamide acidfor processing semiconductors capable of transforming into a polyimideresin having imide rings and/or benzoylenequinazolone rings useful asinsulating films for semiconductors such as surfaceprotecting film ofsemiconductors or layer-insulating film of semiconductors havingmulti-layer construction that does not have the above-mentioned defects.

The inventors have found that the object may be attained by controllingthe manufacturing process of the polyamide acid obtained from a diamineand a tetracarboxylic acid dianhydride optionally with adiaminocarbonamide, as well as concentration of the polyamide acid inthe solution and the viscosity of the solution.

Heretofore, cyclized rubber series photoresists have been known asexamples of forming thin films of less than 5 μM thick without pin holeson a wafer, with high accuracy, by the rotatory coating method.

It has become possible to form a uniform resin film having a superiorheat-resistance by rotatory-coating, on a wafer, using the polyamideacid obtained by the method of the invention.

The method of the invention comprises firstly reacting a diamine and atetracarboxylic acid dianhydride optionally with a diaminocarbonamide,in an inert solvent at below 40° C. for 30 minutes to 24 hours, untilthe reduced viscosity at 30° C. reaches above 0.5 dl/g. Preferably above0.7 dl/g., and secondly adjusting the reduced viscosity to more than 0.3dl/g. preferably 0.35 to 0.45 dl/g., and the viscosity at 25° C. of thesolution to 500-3,000 centipoise (cps) preferably 800-1500 cps byheating at 50°-100° C. for 30 minutes to 24 hours.

DETAILED DESCRIPTION OF THE INVENTION

The polyamide acid obtained by the method of the invention is acopolymer represented by the formulae: ##STR1## (wherein, Ar₁, Ar₂, Ar₃and Ar₄ each represent an aromatic residue and m, n and k each representas integer) which, in turn, is transformed by dehydration andcyclization into a polyimide group resin containing a group representedby the formulae: (wherein, Ar₁, Ar₂, Ar₃ and Ar₄ each represent anaromatic residue).

The diamines to be employed in the invention include, for example,4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenylmethane,4,4'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl sulfide, benzidine,m-phenylenediamine, p-phenylenediamine, 1,5-naphthalenediamine and2,6-naphthalenediamine. They may be used alone or in combination.

The tetracarboxylic acid dianhydride include, for example, pyromelliticacid dianhydride, 3,3', 4,4'-diphenyltetracarboxylic acid dianhydride,3,3',4,4'-benzophenonetetracarboxylic acid dianhydride,cyclopentanetetracarboxylic acid dianhydride,1,2,5,6-naphthalenetetracarboxylic acid dianhydride,2,3,6,7-naphthalenetetracarboxylic acid dianhydride,2,3,5,6-pyridinetetracarboxylic acid dianhydride,1,4,5,8-naphthalenetetracarboxylic acid dianhydride,3,4,9,10-perylenetetracarboxylic acid dianhydride and4,4'-sulfonyldiphthalic acid dianhydride. They may be used alone or incombination.

The diaminocarbonamide compounds include, for example,4,4'-diaminodiphenyl ether-3-carbonamide, 3,4'-diaminodiphenylether-4-carbonamide, 3,4,'-diaminodiphenyl ether-3'-carbonamide,3,3'-diaminodiphenyl ether-4-carbonamide,4,4'-diaminodiphenylmethane-3-carbonamide,3,4'-diaminodiphenylmethane-4-carbonamide,3,4'-diaminodiphenylmethane-3'-carbonamide,3,3'-diaminodiphenylmethane-4-carbonamide, 4,4'-diaminodiphenylsulfone-3-carbonamide, 3,4'-diaminodiphenyl sulfone-4-carbonamide,3,4'-diaminodiphenyl sulfone-3'-carbonamide, 3,3'-diaminodiphenylsulfone-4-carbonamide, 4,4'-diaminodiphenyl sulfide-3-carbonamide,3,4'-diaminodiphenyl sulfide-4-carbonamide, 3,3'-diaminodiphenylsulfide-4-carbonamide, 3,4'-diaminodiphenyl sulfide-3'-carbonamide and1,4-diaminobenzene-2-carbonamide. They may be used alone or incombination.

In preparing the polyamide acid from the abovementioned compounds, aninert organic solvent is used. Particularly preferable are those whichdissolve the polyamide type intermediate formed. They include, forexample, N-methyl-2-pyrrolidone, N,N-dimethylacetamide,N,N-dimethylformamide, N,N-diethylformamide, dimethyl sulfoxide,hexamethylphosphoramide and tetramethylenesulfone. They may be usedalone or in combination.

The preparation of polyamide acid according to the invention comprisestwo steps, namely the first step to synthesize a polyamide acid of highmolecular weight, and the second step to reduce the molecular weight tothe desired degree and viscosity by heating the thus obtained polyamideacid of high molecular weight, which is a so-called cooking process.

In the first step, it is required to polymerize the polyamide acid tothe extent that the reduced viscosity at 30° C. reaches more than 0.5dl/g. An insufficient polymerization of polyamide acid may cause aninsufficient heat-resistance of the cured film finally obtained, aninsufficient extent of second step (cooking process) to reduce themolecular weight to obtain the aimed viscosity, or runaway of coatedfilm due to short period for the second step.

The second step, or cooking process, is performed in order to reduce theviscosity of the polyamide acid solution obtained in the first step andhaving a high viscosity to an extent that the solution may be rotatorycoated with a photoresist rotatory machine and to prevent the runaway ofcoated film by singledispersing the molecular weight distribution of thepolyamide acid. In the second step, the polyamide acid solution isadjusted to obtain the reduced viscosity of more than 0.3 dl/g. at 30°C. and polyamide acid solution viscosity of from 500 to 3,000 cps at 25°C.

In this invention, the above-mentioned figures must be maintainedbecause the film may not sufficiently be polymerized by heating, and asa result, the heatresistance of the cured film is impaired, if themolecular weight of the polyamide acid is so reduced that the reducedviscosity becomes less than 0.3 dl/g. At that time, the viscosity ofpolyamide acid solution at 25° C. must be maintained at 500 to 3,000 cpsbecause a film of more than 1μ thick may not be obtained by coating thesolution on a wafer with a photoresist rotatory coating machine, if theviscosity is below 500 cps. On the contrary, the thickness of marginalpart becomes greater than that of the central part even at a highrotating speed of the photoresist rotatory coating machine due to thehigh viscosity, and therefore, the yield of semiconductor becomes low,if the viscosity is above 3,000 cps.

In this invention, it is preferable that the concentration of polyamideacid ranges from 5 to 30% by weight. The thickness of cured film may notreach the desired value and the uniformity of coated film may not beattained for obtaining the desired thickness by reducing the rotatingspeed of the machine, if the concentration is below 5% by weight. On thecontrary, if the concentration of polyamide acid is above 30% by weightand if the polymerization is so performed that the reduced viscosity isabove 0.5 dl/g., the stirring efficiency is lowered due to the highviscosity and the synthesis becomes, in effect, impossible.

The preparation of polyamide acid according to the invention isperformed, for instance, as follows.

In the first step, a diamine optionally with a diaminocarbonamide aredissolved in an inert solvent and a tetracarboxylic acid dianhydride isadded. The mixture is stirred at below 40° C., preferably around orbelow room temperature. The reaction proceeds rapidly and the viscosityof the reaction system increases gradually, forming a polyamide acid ofhigh molecular weight. It is preferable to minimize the free acidcontent in the tetracarboxylic acid dianhydride, for example, byrecrystallizing it from acetic anhydride in order to obtain a polyamideacid having such a molecular weight that the reduced viscosity at 30° C.is above 0.5 dl/g.

The purification also attains removal of metal ions which give thepolyamide acid bad electrical influence when it is used as an insulatingfilm for a semiconductor. Similarly, it is preferable that the diamineand the diaminocarbonamide to be used are recrystallized from, forexample, an alcohol group solvent to remove metal ions.

It is also preferable that the inert solvent to be used is purified bydistillation to remove metal ion impurities and water.

The polyamide acid thus obtained having the reduced viscosity at 30° C.of above 0.5 dl/g. is adjusted in the second step to have theabove-mentioned reduced viscosity and solution viscosity, by heating thereaction system at 50°-φ° C. and hydrolyzing the polyamide acid of highmolecular weight with trace amount of water present in the reactionsystem, reducing the molecular weight.

In the preparation of polyamide acid from a diamine and atetracarboxylic acid dianhydride optionally with a diaminocarbonamide,it is preferable that approximately equal molar amount of atetracarboxylic acid dianhydride is used to the amount of diamine or tothe total amount of diamine and diaminocarbonamide in order to obtainthe best heat-resistance.

The measurement of reduced viscosity is briefly explained as follows.The polyamide acid solution is poured into ice-water, methanol or thelike giving a precipitate, which is thoroughly triturated, separated byfiltration and dried. The resulting powder is dissolved in an organicsolvent, the concentration is adjusted and subjected to measurement witha Ubbelohde's viscometer.

The viscosity of polyamide acid solution may be measured by aconventional technique with, for example, a Brookfield type viscometer.

The process of forming a thin film by coating the polyamide acid,obtained by the method of the invention, on a semiconductor is brieflyexplained as follows. A wafer useful in many semiconductor apparatus isset in a photoresist rotatory machine, and the polyamide acid solutionis dropped onto the wafer by appropriate equipment such as a squirtdevice. The wafer is then rotated to coat the polyamide acid solutionuniformly on the entire surface of the wafer.

The number of rotations is preferably adjusted to 3,000-6,000 rpm. Thewafer is then heat-processed at above 300° C. for about one hour to forma cured film on the wafer. It is preferable that the heatprocessing isperformed by stepwise increasing the temperature from a low temperature,for example, about 100° C. to obtain a coated film with no pin holes andno residual distortion.

An applicational example of the polyamide acid obtained by the method ofthe invention as an layerinsulating film of an integrated circuit(IC)having a double layer wiring construction is briefly explained asfollows. On a substrate having the first layer wiring formed, thepolyamide acid solution is rotatory-coated and heated to form a curedfilm, according to the above-mentioned process.

Then, the cured film is etched selectively with a photoresist to formthrough holes(interlayer-connecting holes). On the substrate, aluminumas the second layer conductor metal is vacuum-deposited, and a wiringlayer is formed by photoetching.

Another application of the polyamide acid obtained by the method of theinvention to the surface-protecting film of transistors for theprevention of the penetration of contaminants is given as follows. Acured film is formed on a transistor substrate having an electrode,according to a similar method as described above. Then the cured film isremoved only where bonding is applied, giving a transistor having asurface-coating film.

The invention is further explained by the following examples. Thepurified materials used in the examples were recrystallized as follows.

(1) Purification of 4,4'-diaminodiphenyl ether

In a 5 liter flask equipped with a reflux condenser, 220 g. ofunpurified 4,4'-diaminodiphenyl ether and 2,840 g. of n-butanol wereplaced. The 4,4'-diaminodiphenyl ether was dissolved under reflux, 60 g.of active carbon was added and the mixture was filtered. The filtratewas allowed to stand at room temperature for 2 days and the precipitateswere collected by filtration. The precipitates were dried under reducedpressure at 40° to 50° C. to yield the purified 4,4'-diaminodiphenylether.

(2) Purification of 4,4'-diaminodiphenyl ether-3-carbonamide

The purification was conducted in the same manner as in (1), by using300 g. of unpurified 4,4'-diaminodiphenyl ether-3-carbonamide, 1,430 g.of ethanol and 1,800 g. of ion-exchanged water as solvents forrecrystallization to give the purified 4,4'-diaminodiphenylether-3-carbonamide.

(3) Purification of p-phenylenediamine

The purification was conducted in the same manner as in (1), by using200 g. of unpurified p-phenylenediamine, 2,560 g. of ethanol and 1,280g. of ethyl ether as solvents for recrystallization to give the purifiedp-phenylenediamine.

(4) Purification of pyromellitic acid dianhydride and of3,3',4,4'-benzophenonetetracarboxylic acid dianhydride.

In a 5 liter flask equipped with a reflux condenser, 600 g. ofunpurified pyromellitic acid dianhydride and 3,560 g. of aceticanhydride were placed. The pyromellitic acid dianhydride was dissolvedunder reflux, 90 g. of active carbon was added and the mixture wasfiltered. The filtrate was allowed to stand at room temperature for 2days and the precipitates were collected by filtration and dried at 150°C. for 2 days, giving the purified pyromellitic acid dianhydride.

The purification of 3,3'4,4'-benzophenonetetracarboxylic aciddianhydride was conducted under all the same conditions to yield thepurified 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride.

Concentrations of free acids in unpurified and purified pyromelliticacid dianhydride and of unpurified and purified3,3',4,4'-benzophenonetetracarboxylic acid dianhydride were 24.57, 0.04,0.86 and 0.00%, respectively.

EXAMPLE 1.

In a 300 ml. three-necked flask equipped with a thermometer, a stirrerand a calcium chloride tube, 7.29 g. (0.03 mol) of above-purified4,4'-diaminodiphenyl ether-3-carbonamide, 6.0 g. (0.03 mol) ofabove-purified 4,4'-diaminodiphenyl ether and 149 g. of above-purifiedN,N-dimethylacetamide were placed and stirred thoroughly, while coolingthe flask in an ice bath.

To the mixture, 13.08 g. (0.06 mol) of above-purified pyromellitic aciddianhydride was added little by little. Two hours after completion ofthe addition, the ice bath was removed and the whole mixture was stirredat room temperature for 5 hours to proceed with the reaction andcomplete the first step.

A portion of the reaction solution was taken out and the reducedviscosity and the solution viscosity of the polyamide acid weremeasured, which turned out to be 1.08 dl/g. (0.1 g./100 ccdimethylsulfoxide solution at 30° C., the measurement conditions beingthe same in hereinafter description ) and 45,100 cps (Brookfield typeviscometer at 25° C., the measurement conditions being the same ashereinafter described), respectively.

The flask was then heated in a water bath to maintain the reactionsystem at 85° C., with stirring, to proceed with the second step(cooking process). The viscosity of the solution was, i.e., lowered,during the step.

1.5 Hours, 4.5 hours and 7 hours after the reaction system reached 85°C., 300 cc each of the reaction solution was taken out from the reactionsystem to obtain three kinds of polyamide acid solution A (cookingprocess, 1.5 hours), B (cooking process, 4.5 hours) and C (cookingprocess, 7 hours). The reduced viscosity and the solution viscosity ofsamples A, B and C were 0.74 dl/g., 2,560 cps; 0.50 dl/g., 1,040 cps;0.41 dl/g. and 760 cps, respectively.

EXAMPLE 2.

To a flask similar to that used in Example 1, 4.37 g. (0.018 mol) ofpurified 4,4'-diaminodiphenyl ether-3-carbonamide, 4.54 g. (0.042 mol)of purified p-phenylenediamine and 254 g. of purifiedN-methyl-2-pyrrolidone were placed. The mixture was heated at 35° C. ina water bath to obtain a homogeneous solution. The water bath wasremoved and the flask was maintained at room temperature. 6.54 g. (0.03mol) of pyromellitic acid dianhydride and 9.66 g. (0.03 mol) of3,3',4,4'-benzophenonetetracarboxylic acid dianhydride were added littleby little, and the whole mixture was stirred for 10 hours to proceedwith the reaction and complete the first step. A portion of the reactionsolution was taken out, and the reduced viscosity and the solutionviscosity of the polyamide acid were measured according to the methodsdescribed in Example 1, which turned out to be 0.96 dl/g. and 5,920 cps,respectively.

The flask was then heated in a water bath to maintain the reactionsystem at 80° C., with stirring, to proceed with the second step(cooking process). The viscosity of the reaction solution was graduallyreduced, i.e., lowered, during the step. 2 Hours and 6 hours after thereaction system reached 80° C., 30 cc each of reaction solution wastaken out to obtain two kinds of polyamide acid solutions D (cookingprocess, 2 hours) and E (cooking process, 6 hours).

The reduced viscosity and the solution viscosity of solutions D and Ewere 0.62 dl/g. and 2,160 cps; 0.46 dl/g. and 1,480 cps, respectively.

Comparison 1

The reaction solution of Example 1 cooked at 85° C. for 7 hours wasfurther cooked at 85° C. for 7 hours to obtain a polyamide acid solutionF (cooking process, 14 hours). The reduced viscosity and the solutionviscosity were 0.27 dl/g. and 450 cps, respectively.

Comparison 2

For the pyromellitic acid dianhydride of the same component as inExample 1, 12.06 g. of purified pyromellitic acid dianhydride and 1.02g. of unpurified pyromellitic acid dianhydride were added little bylittle. 2 Hours after addition, the ice bath was removed and thereaction mixture was stirred for further 3 hours at room temperature toproceed with the reaction, giving a polyamide acid solution G. Thereduced viscosity and the solution viscosity were 0.32 dl/g. and 880cps, respectively.

The thermal weight loss rate of cured film, film thickness when coatedon a wafer, and conditions of coated film were examined with respect tothe polyamide acids obtained in Examples and Comparisons, and theresults are shown in the following Table.

                                      TABLE                                       __________________________________________________________________________                                 Compar-                                                                            Compar-                                                   Example 1                                                                              Example 2                                                                           ison 1                                                                             ison 2                                                    A  B  C  D  E  F    G                                           __________________________________________________________________________    First                                                                              Reduced viscosity                                                                      1.08     0.96  1.08 0.32                                        step (dl/g., 30° C.)                                                        Solution viscosity                                                                     45,100   5,920 45,100                                                                             880                                              (cps, 25° C.)                                                     Second                                                                             Reduced viscosity                                                                      0.74                                                                             0.50                                                                             0.41                                                                             0.62                                                                             0.46                                                                             0.27 --                                          step (dl/g., 30° C.)                                                        Solution viscosity                                                                     2,560                                                                            1,040                                                                            760                                                                              2,160                                                                            1,480                                                                            450  --                                               (cps, 25° C.)                                                     Resin concentation (wt. %)                                                                  14.6                                                                             14.8                                                                             14.9                                                                             8.6                                                                              8.8                                                                              15.1 14.7                                        Weight loss rate                                                                            3.33                                                                             3.87                                                                             4.20                                                                             6.09                                                                             8.27                                                                             35.43                                                                              91.02                                       (450° C. 5 hrs., in air)                                               Film thickness (μm)                                                                      1.69                                                                             1.32                                                                             1.20                                                                             1.62                                                                             1.47                                                                             0.99 1.11                                        Conditions of coated film                                                                   good                                                                             good                                                                             good                                                                             good                                                                             good                                                                             good Cracks                                                                        observed                                    __________________________________________________________________________

Various properties indicated in the Table were measured as follows:

Reduced viscosity

1 g. of a polyamide acid solution was poured into icewater andprecipitated. The precipitates were pulverized with a liquidizer,separated by filtration, washed thoroughly with distilled water anddried in vacuo. The thus obtained powder was dissolved indimethylsulfoxide to make a 0.1 dl/g. solution, of which the viscositywas measured with a Ubbelohde's viscometer in accordance with JapaneseIndustrial Standard K 5400.

Solution viscosity

Measured with a Brookfield type viscometer in accordance with JapaneseIndustrial Standard K 6901.

Resin concentration

Measured in accordance with Japanese Industrial Standard C 2103. Sampleweight, 1.5 g; heating at 200° C. for 2 hours.

Weight loss rate

The polyamide acid solution was drawn onto a glass plate and heated at100° C. for 1 hour, then at 200° C. for 1 hour and at 350° C. for 1 hourto prepare a film, of which the weight loss rate was measured afterheating at 450° C. for 5 hours in air.

Film thickness

On a 40 φ silicon wafer set on a photoresist rotatory coating machine,about 5 g. of polyamide acid solution was dropped and rotatory-coatedfor 30 seconds at 4,000 rpm. A cured film was formed on the wafer byheat-processing at 100° C. for 1 hour, then at 200° C. for 1 hour and at350° C. for 1 hour. The film thickness was measured with a talystep.

Conditions of coated film

Conditions of coated film on the wafer was observed visually through amicroscope.

As shown in the foregoing Examples, uniform insulating films having nopin holes caused by runaway and the like and having a goodheat-resistance sufficiently with standing heat processing uponpreparation of semiconductor apparatus may be obtained and thereliability of semiconductor apparatus may be largely improved byapplying the polyamide acid obtained by the method of the invention tosemiconductor apparatus.

We claim:
 1. A method of preparing polyamide acid useful for processingof semi-conductors from (i) at least one diamine and (ii) atetracarboxylic acid dianhydride and a diaminocarbonamide in an inertsolvent, which comprises the steps of(1) reacting (i) at least onediamine and (ii) at least one tetracarboxylic acid dianhydride and atleast one diaminocarbonamide, at a temperature below 40° C. to form apolyamide acid having a reduced viscosity above 0.5 dl/g at 30° C., and(2) heating said polyamide acid in an inert solvent at a temperaturebetween 50° C. and 100° C. to lower the viscosity of said polyamide acidwhereby the reduced viscosity of said polyamide acid is more than 0.3dl/g at 30° C. and the solution viscosity of said polyamide acid in theinert solvent is 500 to 3,000 centipoises at 25° C.,the reducedviscosity being measured by forming a 0.1 dl/g solution of saidpolyamide acid in dimethylsulfoxide and measuring the viscosity of thesolution with a Ubbelohde's viscometer in accordance with JapaneseIndustrial Standard K 5400, and the solution viscosity being measured bymeasuring the viscosity of the solution of said polyamide acid indimethylsulfoxide with a Brookfield type viscometer in accordance withJapanese Industrial Standard K
 6901. 2. The method of claim 1, whereinthe reaction of said diamine to form said polyamide acid is carried outuntil said reduced viscosity at 30° C. reaches above 0.7 dl/g in thefirst step of said method, and wherein said polyamide acid is heated tolower the reduced viscosity to between 0.35 and 0.45 dl/g and thesolution viscosity to between 800 and 1,500 centipoises at 25° C. insaid second step of said method.
 3. The method of claim 2, whereinsaiddiamine is selected from the group consisting of 4,4'-diaminodiphenylether, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl sulfone,4,4'-diaminodiphenyl sulfide, benzidine, m-phenylene-diamine,p-phenylenediamine, 1,5-naphthalenediamine and 2,6-naphthalenediamine;said tetracarboxylic acid dianhydride is selected from the groupconsisting of pyromellitic acid dianhydride,3,3'4,4'-diphenyltetracarboxylic acid dianhydride,3,3',4,4'-benzophenonetetracarboxylic acid dianhydride,cyclopentanetetracarboxylic acid dianhydride,1,2,5,6-naphthalenetetracarboxylic acid dianhydride,2,3,6,7-naphthalenetetracarboxylic acid dianhydride,2,3,5,6-pyridinetetracarboxylic acid dianhydride,1,4,5,8-naphthalenetetracarboxylic acid dianhydride,3,4,9,10-perylenetetracarboxylic acid dianhydride and4,4'-sulfonyldiphthalic acid dianhydride; and said diaminocarbonamide isselected from the group consisting of 4,4'-diaminodiphenylether-3-carbonamide, 3,4'-diaminodiphenyl ether-4-carbonamide,3,4-diaminodiphenyl ether-3'-carbonamide, 3,3'-diaminodiphenylether-4-carbonamide, 4,4'-diaminodiphenylmethane-3-carbonamide,3,4'-diaminodiphenylmethane-4-carbonamide,3,4'-diaminodiphenylmethane-3'-carbonamide,3,3'-diaminodiphenylmethane-4-carbonamide, 4,4'-diaminodiphenylsulfone-3-carbonamide, 3,4'-diaminodiphenyl sulfone-4-carbonamide,3,4'-diaminodiphenyl sulfone-3'-carbonamide, 3,3'-diaminodiphenylsulfone-4-carbonamide, 4,4'-diaminodiphenyl sulfide-3-carbonamide,3,4'-diaminodiphenyl sulfide-4-carbonamide, 3,3'-diaminodiphenylsulfide-4-carbonamide, 3,4'-diaminodiphenyl sulfide-3'-carbonamide and1,4-diaminobenzene-2-carbonamide.
 4. The method of claim 2, wherein4,4'-diaminodiphenylether, 4,4'-diaminodiphenylether-3-carbonamide,pyromellitic acid dianhydride and 3,3',4,4'-benzophenonetetracarboxylicacid dianhydride are reacted in the reaction step (1).
 5. The polyamideacid produced by the process of claim
 2. 6. The polyamide acid producedby the process of claim
 3. 7. The polyamide acid produced by the processof claim 4.